In the telephone art, several different electrical signals are transmitted over the pair of wires that connects each telephone instrument to the central office. In addition to the audio voice signal, there is the off-hook signal which indicates that the telephone instrument is in use, the dialing signal which transmits the telephone number dialed, and the ringing signal which causes an alerting device to operate. Since the original alerting devices were bells, that signal is known as a ringing signal. In order to distinguish it from the various other signals, the ringing signal is a high voltage alternating current signal with a frequency of approximately 20 Hz. Various circuits have been devised which reliably distinguish valid ringing signals from among the other signals and the many types of noise that can exist on a telephone line. A relatively recent development, however, is what is known as distinctive ringing in which short bursts of ringing signal are applied to the alerting device to indicate the source of the call. For example, ringing in single bursts might indicate that a call has originated from a long distance line, while groups of two bursts may indicate that the call has originated within the local system. This distinctive ringing has been available within the confines of a private branch exchange (PBX) which serves a single location. Because the pre-existing ringing detectors could not reliably detect these distinctive ringing signals, a new detector was devised which operated very well within the confines of a PBX system. U.S. Pat. No. 4,491,691 which issued to Embree et al. on Jan. 1, 1985 describes such a ringing detecting system.
According to the Embree et al. distinctive ringing detector, a digital magnitude comparator produces a binary output indicative of whether the tip-ring voltage, that is, the telephone line voltage, exceeds a predetermined magnitude. The integrator generates a time integral of the digital output; when the time integral exceeds a predetermined value, ringing is indicated.
Because of the success of the distinctive ringing feature with PBX's, a demand has arisen to provide the same service with off premise stations. Unfortunately, the environment for off-premise telephone plant is much harsher, rendering even the Embree ringer unsatisfactory for this purpose. The inductance associated with long lines together with much greater interference pickup make both dial pulses and interferences such as switching transients and lightning resemble much more closely the short bursts of distinctive ringing. In addition, the Embree circuit requires a power dissipating bridge which lowers the tip-ring impedance to an undesirable level.
Our invention provides much more accurate detection of ringing signals in the off-premise environment and eliminates the need for a low impedance power dissipating bridge.